Why Some Kids Are Smarter

The brains of more intelligent children appear to develop in a characteristic way, growing quickly over an extended period between the ages of 5 and 12. These findings – some of the most detailed research on brain development and IQ – resulted from a 15-year study done by the National Institutes of Mental Health (NIMH.)

The study, which used magnetic resonance imaging (MRI) to gain a detailed picture of how the brains of children change over time, found that in kids who did better on standard IQ tests, the cortex grew thicker and faster and its growth peaked later than among their average peers.

Researchers say the findings could help scientists pinpoint genes involved in brain development and IQ levels. It could also give scientists a better picture of normal brain development and shed light on childhood developmental and psychiatric diseases, such as Attention Deficit Hyperactivity Disorder (ADHD.)

“The study shows that there are clear features in brain images that are different for smart children versus average children,” says Paul Thompson, a brain-imaging expert at the University of California, Los Angeles (who was not involved in the project). “Now it should become extremely fast to identify some of the main factors that make the brain develop in a healthy way…such as how kids are taught, their diet, their parental upbringing, or…genetics.”

Most previous studies of brain development have compared children’s brains at a single time in their lives. The new study, published today in the journal Nature, took a broader look, by studying children as they grew. The researchers followed more than 300 children, ages 5 to 19, for a total of 15 years, taking MRI images of their brains at several different times in their development.

The cortex, which is made up of nerve-cell bodies covering the outer layer of the brain, is largely responsible for higher-order brain functions, such as reasoning and perception. All children show the same basic pattern of cortical development: the cortex grows during childhood, and thins in adolescence, as unused neural connections are pruned away.

According to this new study, children with the highest IQs start out with a thinner cortex, which undergoes rapid growth, peaking at around age 12, instead of age 8 or 9 for children who got average scores on IQ tests. “Children with the most agile minds have the most rapidly changing cortex,” says Philip Shaw, a child psychiatrist at NIMH who led the research.

The greatest correlation was seen in the prefrontal cortex, which mediates planning and complex reasoning. “Intelligent children have a prolonged period of thickening in the prefrontal cortex. We wonder if this gives them an extended period to develop the complex circuitry to support high-level thought,” says Shaw.

John Gabrieli, a neuroscientist at MIT, says the long-term duration of the study allowed scientists to uncover patterns not observable in previous studies, such as the delayed developmental trajectory for people who go on to score high on IQ. “If you tested just at age 7 or 14, you would have come to opposite conclusions,” he says.

“It would be interesting to see how those trajectories evolve in kids with dyslexia or other learning disorders,” Gabrieli adds. “Perhaps you could identify children at risk for learning disorders much earlier than we now do.”

Shaw’s team recently finished a similar study of children with ADHD, in which researchers looked for the brain changes that accompany spontaneous remission of the learning disorder. Shaw says the database of normal brain development helped interpret those findings, which they expect to release in a few months.

Richard Haier, a neuroscientist at the University of California, Irvine, who studies IQ, says the findings also raise the question: “What influences the development of this brain area to give some people a thicker cortex than others? Is it based on genetics or some interactions or experience that a young person has that may foster a thicker cortex?”

Shaw and his colleagues now plan to search for genetic variants that are linked to the pattern of cortical development, and hence IQ. Many studies have demonstrated that IQ has a strong genetic component, yet it’s still unclear how those genetic differences manifest in the brain.

While this area of research has been contentious – critics worry that knowing genetic determinants of IQ could lead to a deterministic attitude toward education – experts say that IQ is most likely linked to a complex interaction of genetics and environment.

Shaw says it’s still too soon to tell how much of the growth pattern is due to genetics and how much to the environment, or both. His team plans to investigate this question in future studies.